[FIX] website_event: fix error in scss

A variable was not used where it should have, breaking the events page
background with the upcoming forum redesign.

closes odoo/odoo#37703

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website_event/static/src/scss/website_event.scss

@@ -93,7 +93,7 @@ $o-wevent-event-title-sizes-variants: (
             }
         }
         &.opt_event_list_cards_bg {
-            @if (color('body') == #FCFCFC) {
+            @if (color('body') == $o-portal-default-body-bg) {
                 @extend .bg-200;
             }
         }